This invention relates to a supercharging pressure control method for internal combustion engines, which controls the supercharger of the engine in a feedback manner responsive to the difference between the actual supercharging pressure and a desired value thereof, and more particularly to a method of this kind, which controls the supercharging pressure in a transient state immediately before and after the start of the feedback control.
In order to control the supercharging pressure in an internal combustion engine for automotive vehicles to appropriate values in accordance with operating conditions of the engine, it has generally been employed to control the operating amount of the supercharger in a feedback manner responsive to the difference between the actual value of supercharging pressure and a desired value thereof. The feedback control is effective to absorb or correct variations in the operating characteristics of the supercharging pressure control system such as actuators used therein, as well as aging changes therein, thereby enabling it to properly control the supercharging pressure.
However, it is also well known that feedback control of supercharging pressure undergoes a time lag inherent thereto, that is, it takes time for the supercharging pressure to be brought to the desired value. Particularly in a turbocharger driven by an exhaust gas flow, such time lag is long, as is also well known.
The time lag in the feedback control causes overshooting or hunting of the supercharging pressure. Particularly, in a transient state such as a state immediately after the supply of supercharging pressure to the engine has been started upon request for acceleration by the driver, the difference between the actual value of supercharging pressure and the desired value thereof is too large for the operating amount of the supercharger to be controlled in quick response to an actual change in the supercharging pressure, resulting in an excessive rise (overshooting) or an excessive drop (undershooting) in the supercharging pressure, hunting thereof, etc.
As the amount of overshooting beyond the desired value becomes larger, the hunting time increases, rendering the supercharging pressure control more unstable. Excessive overshooting can cause overboosting, resulting in knocking of the engine and hence degraded driveability.
To overcome the above disadvantages with the conventional feedback control, it has been proposed, e.g. by Japanese Provisional Patent Publication (Kokai) No. 61-164042, to provide two different control modes, one of which is a feedback control mode applied in a steady region where the supercharging pressure is stable, and the other is an open loop control mode applied in a transient region where the supercharging pressure is unstable, and the supercharging pressure is controlled selectively in either of the control modes depending upon which of the two regions the engine is operating in, to thereby control the supercharging pressure to the desired value with high responsiveness and high accuracy.
According to the proposed method, whether or not the engine is operating in the feedback control region is determined based upon a parameter representative of load on the engine, such as intake pressure within the intake pipe. Specifically, if the engine load parameter value is higher than a predetermined value, it is determined that the engine is operating in the feedback control region. Then, the supercharging pressure is controlled in the feedback control mode.
However, the proposed method still has room for further improvement, as stated below:
First, if the intake pressure within the intake pipe is employed to discriminate between the feedback control region and the open loop control region, there are cases where the same control mode should not be applied even when the intake pressure assumes the same value, depending upon the changing manner of intake pressure before the same value is reached. For example, if the same feedback control gain is applied in both cases such that the intake pressure gently rises and the intake pressure rapidly increases, overshooting or hunting can take place upon transition from the open loop control region to the feedback control region, in the latter case.
Secondly, the intake pressure also varies under the influence of other factors, i.e. the operating condition of the engine, and the environmental conditions of the engine such as ambient air temperature and atmospheric pressure. If these factors are ignored in setting the timing of starting the feedback control, the feedback control is started at too early a timing or at too late a timing. If the feedback control is started at too early a timing, the difference between the desired and actual values of supercharging pressure is still large at the start of the feedback control, resulting in overboosting or hunting; whereas if the feedback control start is made at too late a timing, the aforementioned variations and aging changes in the operating characteristics of the actuators cannot be corrected for a long time before the feedback control is started.
Thirdly, conventionally, the supercharging pressure or intake pressure is used for discrimination between the feedback control region and the open loop control region such that when the supercharging pressure is higher than a predetermined value which is lower than the desired value, and accordingly is deemed to be in a steady state, the feedback control is effected. However, when the supercharging pressure rises at a high rate, if the feedback control is started immediately when the supercharging pressure exceeds the predetermined value, the starting of the feedback control will cause overshooting or hunting of the supercharging pressure. A way to overcome this disadvantage would be to set the above-mentioned predetermined value of supercharging pressure at a relatively high value closer to the desired value. However, there is the possibility that the supercharging pressure is brought into a steady state even when it is still lower than the predetermined value, so that the feedback control is not started.
Fourthly, since the intake pressure or supercharging pressure also varies in response to the throttle valve opening, the operating amount of the supercharger has to be controlled to different values depending upon the actual value of the throttle valve opening to maintain the same supercharging pressure. However, according to the conventional methods, even if the throttle valve is not almost fully open (the throttle valve opening is below a predetermined value), the engine is deemed to be in the steady state when the supercharging pressure exceeds a predetermined value, and then the feedback control is started. When the throttle valve becomes almost fully open thereafter, the operating amount is largely changed so as to bring the supercharging pressure to the desired value. Due to the large change in the supercharging pressure as well as to the aforementioned inherent time lag in the feedback control system, the operating amount cannot be promptly brought to a value which can maintain the supercharging pressure at the desired value, resulting in overshooting or hunting of the supercharging pressure. A way to overcome the disadvantage would be to deem that the supercharging pressure is in a steady state when the throttle valve is almost fully open, and then start the feedback control. However, according to this way, the supercharging pressure can be deemed to be in a steady state even when the actual supercharging pressure has not been increased to a sufficient level, and then the feedback control is started even though the supercharging pressure is still in a transient state. This results in delayed rising of the supercharging pressure and degraded control responsiveness.